charged amino acid
glutamic acid
3 types of substitutions mutations
- silent - no change in aa code
- missense - still code for an amino acid, but not necessarily the right one
- nonsense - change an amino acid codon into a stop codon
what are insertions or deletions
additions or losses of nucleotide pairs in a gene
- have a disastrous effect on the resulting protein more often than substitutions
- may alter the reading frame, producing a frameshift mutation
control of gene expression in prokaryotes
operons = A cluster of functionally related genes can be under coordinated control by a single on-off “switch” - the switch = OPERATOR - generally found within promoter
2 types of regulation of gene expression in prokaryotes
positive and negative
whats negative regulation of gene expression
uses 2 types of repressors that block RNA polymerase
- repressible - usually on (trp operon)
- inducible - usually off (lac operon)
repressible type of repressor
e.g. trp operon
- generally on
- anabolic metabolism
- compressor helper - tryptophan
whats in every lactose
1,6 allolactose
inducible type of repressor
e.g. lac operon
- generally off
- catabolic metabolism
- inducer helper - allolactose
whats positive regulation of gene expression in prokaryotes
- CAP) = an activator of transcription - stimulatory protein
- When glucose is scarce, CAP is activated by binding with cyclic AMP
- Activated CAP attaches to the promoter of the lac operon + increases the affinity of RNA polymerase, thus accelerating transcription
what happens when glucose levels are low
cyclic AMP levels are high - high levels of transcription
what happens when glucose levels are high
cyclic AMP levels are low - low levels of transcription
DNA organization
DNA -> HISTONES -> NUCLEOSOMES -> CHROMATIN
control of gene expression in eukaryotes
- regulation of chromatin structure
- histone modification
- DNA methylation
regulation of chromatin structures
- genes can be packed into EUCHROMATIN (open / on) + HETEROCHROMATIN (closed / off)
histone modifications
- histone acetylation: acetyl groups are attached to positively charged lysines in histone tails - loosens chromatin structure - promotes initiation of transcription
- methylation: addition of methyl groups can condense chromatin
- phosphorylation: addition of phosphate groups next to a methylated amino acid can loosen chromatin
- HISTONE CODE HYPOTHESIS - specific combinations of modifications help determine chromatin configuration and influence transcription
DNA methylation
add methyl groups to certain C bases
- can cause long-term inactivation of genes
- it is transgenerational
regulation of transcription initiation
CONTROL ELEMENTS = segments of non-coding DNA that bind transcription factors
- ENHANCERS = may be far away from a gene or even located in an intron
- PROXIMAL CONTROL ELEMENTS = located close to the promoter
- activator - binds to an enhancer and stimulates transcription of a gene
- repressors - inhibiting expression of a particular gene
Combinatorial control of gene activation
mechanisms of post transcriptional gene regulation
allow a cell to fine-tune gene expression rapidly in response to environmental changes e.g. Gcap, splicing, mRNA degradation (mRNA can exist in the cytoplasm for long time)
imitation of translation
mRNAs can be blocked by:
- RNA structure
- proteins
- other RNAs
protein processing and degradation
After translation, various types of protein processing e.g. cleavage + the addition of chemical groups, are subject to control
- PROTEASOMES = giant complexes that bind protein molecules and degrade them
2 types of RNAs that alter gene expression
- Micro RNAs (miRNAs) - bind to mRNA and degrade it / block its translation
- Small intefering RNAs (siRNAs) - bind to chromatin - form heterochromatin
